Embryonic Development of the Cardiovascular System

Necessity and Early Development of the Cardiovascular System

  • The cardiovascular system is the first functional system to develop in the embryo.
  • Its early establishment is dictated by the method through which the embryo receives nutrition.
  • In the earliest phases of development, the embryo relies on diffusion from fluid secreted by uterine glands.
  • As the embryo rapidly increases in size and structural complexity, the diffusion-based system of nutrition becomes insufficient.
  • This creates an urgent necessity for the development of a circulatory system to support continued growth.

Embryological Origins and Tissue Development

  • The cardiovascular system (comprising both blood and blood vessels) originates from the Mesoderm.
  • The specific lineage follows this path: Mesoderm → mesenchyme (pluripotent cells) → angioblastic tissue (hemangioblasts) → cardiovascular system.
  • Angiogenesis is the formal term used to describe the process of blood vessel development.
  • Cardiogenic Field:
    • This is a horseshoe-shaped structure situated around the anterior and lateral portions of the neural plate.
    • It is an area of blood-forming cavities within the visceral mesoderm.
    • These cavities eventually coalesce to form the primitive heart, blood vessels, and blood cells.

Embryonic Folding and Heart Positioning

  • Antero-posterior (cranio-caudal) folding of the embryonic disc significantly alters the position of the developing heart.
  • The cardiogenic field, originally located anteriorly, is brought caudo-ventrally by this folding process.
  • The developing heart tube is eventually positioned ventral to the dorsal aortae.
  • Fusion Processes:
    • The caudal portion of the cardiogenic tube fuses with the cranial portion of the vitelline veins.
    • The caudal portions of the dorsal aortae fuse.
    • The two sides of the cardiac tube fuse into a single primitive heart tube.

The Primitive Heart Tube and Adult Derivatives

  • By approximately 1818 days, the primitive heart tube is organized into distinct regions from head (cranial) to tail (caudal):
    • Truncus arteriosus
    • Bulbus cordis
    • Primitive ventricle
    • Primitive atrium
    • Sinus venosus
  • Following loop formation, these five regions develop into specific recognizable structures in the fully developed heart:
    • Primitive Atrium: Divided into the anterior portions of the left and right atria.
    • Primitive Ventricle: Develops into most of the left ventricle.
    • Bulbus Cordis: Develops into the right ventricle, the conus cordis, and the truncus arteriosus.
    • Truncus Arteriosus: Divided into the outflow tracts of the aorta and the pulmonary trunk.
    • Sinus Venosus: Develops into the posterior portion of the right atrium, the sinoatrial (SA) node, and the coronary sinus.
  • Defects in the development of the aortic arches lead to defects in the great arteries of the body.

Comparative Cardiac Anatomy

  • Fish: Possess a 22-chambered heart consisting of one atrium and one ventricle, with circulation passing through gills.
  • Amphibians: Possess a 33-chambered heart, typically leading to the mixing of oxygenated and deoxygenated blood.
  • Reptiles (e.g., Turtles): Possess a 33-chambered heart that is septated, representings an intermediate stage toward full separation.
  • Birds and Mammals: Possess a fully partitioned 44-chambered heart, ensuring complete separation of oxygenated and deoxygenated blood.

Partitioning of the Atrio-Ventricular Canal and Atria

  • The development of the four-chambered heart requires the partitioning of the common atrium and ventricle.
  • Sequence of Atrial Partitioning:
    • Septum Primum (Primary Atrial Septum): Grows toward the endocardial cushions.
    • Ostium Primum (Foramen Primum): An opening that allows the initial passage of blood from the right to the left atrium as the primary septum grows.
    • Ostium Secundum (Foramen Secundum): Forms in the upper portion of the septum primum as the ostium primum closes, ensuring continued right-to-left blood flow.
    • Septum Secundum (Secondary Atrial Septum): Grows to the right of the primary septum.
    • Foramen Ovale: The passage between the primary and secondary septa. The primary septum (septum primum) acts as a one-way valve for the foramen ovale, preventing blood from returning to the right atrium.
  • Ventricle Partitioning:
    • The interventricular septum comprises a muscular portion and a membranous portion.
    • The interventricular septum eventually fuses with the fused endocardial cushions.

Development of the Great Vessels

  • The truncus arteriosus is divided into the ascending aorta and the pulmonary trunk.
  • This division is achieved by a spiral aortico-pulmonary septum.
  • Significance of the Spiral Formation:
    • The spiral shape ensures that blood from the right ventricle flows into the pulmonary trunk.
    • It ensures blood from the left ventricle flows into the ascending aorta.
  • Transposition of the Great Vessels:
    • If the septum forms straight rather than spiraled, the vessels are transposed.
    • Deoxygenated blood from the right ventricle flows into the aorta, and oxygenated blood from the left ventricle flows into the lungs.
    • This anomaly is incompatible with life.

Fetal Circulation and Physiological Shunts

  • Fetal circulation is designed for the in utero aqueous environment where biological functions (oxygenation and waste removal) are performed by the placenta rather than the lungs or liver.
  • Three Essential Blood Shunts:
    1. Ductus Venosus: A vein that connects the umbilical vein directly to the inferior vena cava. This allows oxygenated blood from the placenta to bypass the liver.
    2. Foramen Ovale: An opening between the right and left atria. It allows blood to bypass the non-functional fetal lungs by moving directly from the right heart to the left heart.
    3. Ductus Arteriosus: A vessel connecting the pulmonary artery to the aorta, which further shunts blood away from the lungs.

Transition to Neonatal Circulation

  • Significant changes occur immediately after birth when the lungs become functional.
  • Pressure Changes:
    • Lungs expand with the first breaths, causing a sudden and dramatic drop in blood pressure within the pulmonary circulation.
    • This pressure shift pushes the primary septum (septum primum) against the secondary septum (septum secundum).
  • Closure of the Foramen Ovale:
    • Physiological Closure: The immediate mechanical pressing of the septa together due to pressure changes.
    • Anatomical Closure: The eventual physical fusion of the two septa.
    • The remnant of the closed foramen ovale is called the fossa ovalis.
  • Adult Remnants of Fetal Structures:
    • Ductus Venosus → Ligamentum venosum (the fibrous remnant in the adult liver).
    • Foramen Ovale → Fossa ovalis (a depression in the wall of the right atrium).
    • Ductus Arteriosus → Ligamentum arteriosum (the fibrous remnant in the adult heart).
  • The liver and lungs, which receive very little blood in the fetus, receive full circulation once these three shunts close after birth.